1. Field of the Invention
The present invention generally relates to liquid level measurement and, more particularly, to the measurement of the quantities of immiscible materials contained within a tank or container.
2. Description of the Prior Art
The measurement of fluids within substantially closed containers has found applications in many fields of endeavor such as fuel level sensing in automobiles and other vehicles. Accordingly, it has long been recognized that the presence of significant amounts of contaminant substances can affect the accuracy of determinations of volume of a fluid from liquid level measurements. In particular, contamination of petroleum-based fuels with water has been a commonly encountered difficulty since fuel tanks must be vented to allow replacement of volumes of fuel withdrawn from a tank with the ambient atmosphere in order to avoid developing a partial vacuum in the tank. The ambient atmosphere may be relatively humid, particularly on water-borne vehicles and the temperature differential between the ambient atmosphere and fuel or the fuel tanks (which will often approximate the water temperature) will cause the moisture in the ambient atmosphere to condense to liquid phase. Therefore, substantial quantities of liquid water may accumulate in fuel tanks over a relatively short period of time.
Since water has a greater specific gravity than most petroleum fuels, such as diesel fuel, water condensing in a fuel tank will generally collect at the bottom of a fuel tank and raise the level of the surface of the fuel in the tank. Since the passage of water into engines is extremely undesirable, numerous techniques have been developed for extracting fuel from a location within a fuel tank which is likely to be uncontaminated with water (e.g. within the volume of the fuel. Since fuels can also contain some other contaminants which would not be harmful if homogeneously distributed in the fuels as charged into a fuel tank, such as solid particles which may settle out of the fuels, any settling of such contaminants will not be collected with fuel as fuel is withdrawn from the tank. Therefore, over several fillings of a tank, a substantial accumulation of such particles, as sludge, may occur and further complicate determination of the volume of usable fuel present in a tank from the level of liquid present in the tank. Principally for this reason, the amount of usable fuel remaining in a tank cannot be adequately extrapolated from a measurement of the changed of level when fuel is charged into the tank and subsequent reductions in liquid levels as fuel is withdrawn.
Further, when it is determined that contaminants should be purged from a tank, it is helpful to be able to ascertain the relative amounts of water and settled solids in order to carry out contaminant removal. For example, when water is removed from the tank, it may be desirable to pass the water through an oil/water separator so that the water may be discharged without release of oil and the oil further processed for possible re-use or disposal. Removal of water from the tank by draining in the presence of excessive amounts of sludge may cause the sludge to mix with the water and could thus adversely affect the operation of such an oil/water separator. Therefore, the relative amounts of contaminants may be determinative of which of a plurality of types of remedial action should be undertaken.
An additional problem may be encountered in tanks carried by vehicles and water-borne vessels, in particular, since the motion of the vehicle may cause shifting of fluids in the tank. Complex wave-like action is often observed in large tanks, both at the liquid surface and at interfaces between immiscible fluids of differing densities. To date, there has been no technique of directly approximating the average fluid levels or location of fluid interfaces without the averaging of a plurality of samples taken over time. Such sampling is time-consuming and expensive since known techniques of measuring fluid interfaces such as oil/water interfaces often involve a single use dip-stick treated with materials which react differently (e.g. produce a color change) which is distinctive of contact with one of these materials. The determination may also be biased by fluid motion (e.g. detection of the highest fluid level during the finite time the dipstick is in place) or otherwise complicated by the removal of fluid from the tank during the period over which samples are taken. Further, for tanks installed in or carried by water-borne vessels, liquid level measurements may be biased by vessel trim.
In instances where fluid level measurement systems are permanently installed in a tank or container, it is most common to use a float on a pivoted arm in order to drive a conductive wiper across a resistive element. However, such arrangements are somewhat unreliable since they may leak or be damaged in a manner which changes the buoyancy of the float. This is especially true if it is attempted to use a float for detection of an interface between immiscible fluids where buoyancy of a float would be very critical due to low differential density of the fluids. Further, such pivoting arms inherently introduce non-linearities into the measurement which may be difficult to compensate. The measurement accuracy available from such mechanical arrangements and other known arrangements is not high due to the friction of the wiper contact with the resistive element and lost motion in the mechanical linkage (e.g. where the float is coupled to a transducer through a pivoted spiral shaft.
Such mechanisms are also sensitive to mounting location and angle and are not suitable for portable use. Further, where there is reason to avoid permanent installation of a fluid level sensor, access to the interior of the tank is likely to be restricted to the diameter of a fill or vent aperture and which may be as little as one inch in diameter. Access may also be restricted by nearby structures such that an elongated device, such as the previously mentioned dip-sticks may be inconvenient or impossible.
Additionally, for a portable liquid level measurement system, the size and geometry of the tank may present particular problems. For example, it may be necessary or desirable to use the same portable liquid level measurement device on tanks or containers of widely disparate sizes, particularly in vertical dimensions. Therefore, the particular sensor hardware may limit the applicability of a particular sensor structure to tanks or containers of particular dimensions. Accordingly in view of the low friction available from rotational bearings and joints, it is difficult to provide a single arrangement of sensor hardware which will accommodate a variety of container or tank dimensions as well as provide sufficiently high resolution in the vertical direction.
Many applications of so-called sonar techniques are also well-known in the art. Sonar techniques involve the generation, by a transducer, of a pulse of ultrasonic energy which may be partially reflected from any surface it encounters. If the propagation speed of the ultrasonic pulse through a medium, such as a fluid, is known or can be approximated, the distance of the surface from the transducer can be approximated. A substantial amount of information can also be derived concerning the nature of the surface from which it is reflected. In particular, in recent years, such systems have become popular equipment for use as depth finders and fish locators on boats and sophisticated graphics displays using display media such as cathode ray tubes or liquid crystal matrices have also been developed and are commercially available with such systems at relatively low cost. An example of such a system is available from APELCO Marine Electronics of 46 River Rd., Hudson, N.H., 0305109922 as Model No. XCD 250.